Tunable external cavity lasers (“ECLs”) are widely used in lightwave test-and-measurement equipment and are becoming recognized as essential components for the rapidly expanding field of wavelength division multiplexed (“WDM”) optical communication. The many applications within this field require many different sets of performance specifications. However, the following are some typical requirements: small form factor of the optomechanical assembly and control system, control over the output laser wavelength, reliability of the laser assembly, and inexpensive fabrication costs.
A known method to selectively tune the central lasing wavelength of an ECL is to place a wedge shaped optical filter in the path of the laser beam. Tuning is obtained by moving the wedge filter across the optical path. Tuning results from the variation in the thickness of the wedged shape filter that intersects with the laser beam. Alternatively, rotating a flat optical filter in the laser beam path will achieve the desired tuning effect by adjusting the path length that the laser beam must traverse through the optical filter.
These methods of tuning an ECL are undesirable, as they require placement of a mechanical actuator, such as a motor, within the ECL module to effect the desired translation or rotation of the optical filter. Placement of a motor within the ECL module can disrupt other sensitive optic elements that include an ECL and generate disruptive electromagnetic interference. Furthermore, the motor is a severe limitation on the need to miniaturize modern ECL modules. Small motors can be relatively expensive and the use of any mechanical device with moving parts tends to be less reliable than stationary mechanical and electronic substitutes.